Free molecular chlorine dioxide in solution is an effective agent for the control of microorganisms and biological film deposits. However, the most common way to produce such solutions of free chlorine dioxide is to use an expensive and complicated chemical generator (see for example U.S. Pat. No. 5,009,875).
British patent 608,068 teaches the use of organic acid anhydrides to convert chlorite anion to free chlorine dioxide at a generally neutral pH. The fire and explosion dangers that result from combining a strong oxidizing chemical, such as sodium chlorite, with an organic chemical are also well known. As a result of the low conversion ratio produced by the compositions of this British patent, and the dangers inherent in compositions combining sodium chlorite and organic chemicals, the compositions of this British patent have received little commercial interest.
Recently, a membrane-type device containing powdered chlorine dioxide precursor chemicals that produce a solution of free molecular chlorine dioxide when the device is immersed in water was described, see WO 99/24356. While this membrane device is superior to the prior art methods of producing chlorine dioxide solutions, the device has some shortcomings. It is relatively expensive (due to the cost of the membrane and of assembly), and the rate of chlorine dioxide delivery can be too slow for certain applications. Also, the device may float on the surface of the water/solution (due to entrapped air or chlorine dioxide gas), and this can result in the loss of some chlorine dioxide to the gas phase. Finally, the preferred membranes are insoluble in water, and may need to be removed from the chlorine dioxide solution after the chlorine dioxide generating reactions are completed. Removal of the spent membrane from the chlorine dioxide solution may, at times, be considered inconvenient.
The prior art also describes attempts to produce chlorine dioxide solutions from solid mixtures, including solid compacts such as tablets and briquettes, that are comprised of materials that will generate chlorine dioxide gas when contacted with liquid water.
U.S. Pat. No. 2,071,094 discloses dry solid compositions, including briquettes, comprised of a soluble chlorite and an acidifying agent that when contacted with water produce a “deodorizing reaction” as the dry composition begins to dissolve (see col. 1, lines 34–38 and col. 2, lines 24–27). Upon reading this patent, it is not clear what is produced when the composition comes into contact with water. The substance, chlorine dioxide, is never mentioned and the only time the word “solution” is used, it is with reference to an aqueous solution of sodium chlorite (see col. 1, line 58). Thus, it cannot be ascertained whether the inventor was attempting to, or indeed ever did, obtain an aqueous solution of chlorine dioxide when his briquette was contacted with water.
U.S. Pat. No. 4,104,190 describes tablets comprising sodium chlorite, a halogen-based activator and a buffer. The tablets may further comprise an acid. The preferred halogen-based activator is sodium dichloroisocyanuric acid. The conversion efficiency reported in the patent for the tablets is, for the most part, low. Ten of the examples in the '190 patent report sufficient data from which it is possible to calculate the chlorine dioxide and chlorite yields. Only two of those examples resulted in a rate of conversion of chlorite to chlorine near or above the minimum of 25% associated with the present invention. The maximum rate of conversion of chlorite to chlorine disclosed in the '190 patent was 37%. The concentration of chlorine produced by the tablets was not disclosed in the '190 patent, but experiments we have carried out have resulted in the conclusion that at the level of near or above 25% conversion of chlorite to chlorine dioxide, undesirably high concentrations of chlorine in solution were also produced. In one of the two examples of the '190 patent, the concentration of chlorine in the product solution was over double that of the concentration of chlorine dioxide in the solution. In the other example of the '190 patent, the concentration of chlorine in the product solution was over four times that of the concentration of chlorine dioxide in the solution.
Since chlorine dioxide is often used because it provides a biocidal function without many of the disadvantages of chlorine, it would desirable to develop a device to produce a highly converted solution of chlorine dioxide that did not also contain excessive amounts of chlorine. It would also be desirable to develop a device that produced a solution of chlorine dioxide where the conversion of chlorite to chlorine dioxide was greater than 37%.
U.S. Pat. No. 5,324,447 describes, inter alia, a tablet comprising a chlorine dioxide precursor (e.g., sodium chlorite) and an activator component (e.g., an organic acid anhydride) that are present in amounts effective to produce (contact) lens disinfecting amounts of chlorine dioxide in a liquid medium (see, col. 3, lines 10–16). The term “disinfecting amount” is defined as such amount as will reduce the microbial burden by one log order preferably in ten (10) minutes or less (see col. 4, lines 11–15). This amount represents very little free chlorine dioxide, as even as little as 2 ppm of free chlorine dioxide can result in a 6 log bacterial reduction in 15 minutes. The patent does not disclose the amount of chlorine dioxide that is generated when a tablet of the invention is dissolved in water. Thus, all of the examples utilize aqueous solutions of stabilized chlorine dioxide and not water to test the tablets.
U.S. Pat. No. 5,399,288 discloses a solid composition releasing chlorine dioxide immediately after dissolution in water (see col. 1, lines 5–7). The composition comprises a chlorite salt, an oxidizing chlorine-releasing agent and a proton donor present in the ratio of 4:1:3 (see col. 1, lines 65–67). When the oxidizing chlorine-releasing agent is omitted from the composition, the final yield of chlorine dioxide obtained was 63% but after three days (see Example 5). Further, and importantly, this patent does not discuss the preparation of tablets (see col. 2, lines 19–21). Thus, it appears that only powdered mixtures of reactants are disclosed.
U.S. Pat. No. 5,719,100 discloses production of chlorine dioxide in an aqueous solution from a tablet comprising a composition of sodium chlorite and an acid activator wherein the composition requires a reaction-preventing barrier between the sodium chlorite, i.e., a protective reactive coat is formed on the sodium chlorite before it is mixed with the acid activator (see col. 4, lines 61–63) and the acid activator such that the two active ingredients do not “explosively react” together prematurely (see col. 4, line 53), i.e., a stable composition is obtained (see col. 4, line 46 through col. 5, line 9).
The present invention provides an improved device in the form of a massive body for the production of chlorine dioxide solutions. This new device rapidly provides high yield solutions of chlorine dioxide and overcomes shortcomings of prior art solid compositions for producing such solutions.